Offset-printable coated white paper having a high fluorescent intensity and method for producing same

ABSTRACT

The invention concerns a coated white sheet having a fluorescence intensity of more than 55, measured in accordance with international standard ISO 11475: 2004, by the difference between the value for the CIE whiteness under D65 illuminant and the value for this same CIE whiteness after interposing a filter that eliminates wavelengths shorter than 420 nm, and being offset-printable with no mottling, said coated sheet comprising at least one base sheet and a printable white surface coat having a pigmented composition which comprises: —at least some white coating pigments and at least one coating binder; —at least one fluorescent whitening agent in a total quantity of 1% dry weight or more with respect to the dry weight of said pigments; —at least one support substance for said whitening agent in a total quantity of dry weight of more than 2% with respect to the dry weight of said pigments. The invention also concerns a method for producing said coated sheet by curtain coating said pigmented composition.

The present invention relates to an offset-printable coated white sheetat least the surface coat of which possesses high fluorescence intensityby dint of its composition. It also relates to a process for itsproduction.

In the field of paper and that of plastic sheets treated by applicationof a pigmented coat to obtain offset-printability, for a long timefluorescent whitening agents, also known as optical brighteners, havebeen used which act by absorbing natural light in the ultraviolet and inthe near-visible below 420 nm then re-emitting the light by fluorescenceat about 440 nm, i.e. in the blue or blue-violet region of the visiblespectrum. They thus augment the whiteness of these coated sheets bygreatly displacing their shades from yellow towards blue.

The use of such fluorescent whitening agents—unsaturated organicmolecules containing two to six sulphone groups—has been widelydescribed, in particular as regards the necessity for such molecules tobe well fixed in the sheet in their trans isomeric form, which is theonly active form.

The best performing existing starting substance by far for ensuring suchbinding is cellulose because of interactions between the electrons ofthe hydroxyl groups and the unsaturated structure of the fluorescentwhitening agent. In fact, it is easy to obtain uncoated papers which arevisually very white by adding a sufficient quantity of an appropriatefluorescent whitening agent; said papers have a high fluorescenceintensity of at least 55 or even of the order of 60 to 70 as measuredusing international standard ISO 11475: 2004.

In the field of offset-printability, it is known that for a good print,in particular for an ink-surface interaction which is as homogeneous aspossible and a shorter ink drying time, it is necessary to apply a coatcomposed of at least one mineral or organic pigment and at least onenatural or synthetic binder to the surface of the base sheet. Dependingon the composition and the application process, the weight of that coatafter drying is at least 5 g/m² dry weight per face in order to perceivean improvement in printability, preferably 10 g/m² per face, and if ahigh level of offset-printability is required, much more, possiblyapplied in several steps.

The problem with such coats is that the fluorescent whitening agents arepoorly fixed by the components of the coats and it is difficult toincrease the fluorescence without causing a counter-whitening effectknown as “greening” which corresponds to a change in the shade of thecoated sheet from blue towards green. This change can be quantified bythe variation Δa* in the trichromatic coordinate a* of the CIELAB spacemeasured under the conditions of standard ISO 11475: 2004.

Another problem is that the coats, as a function of their weight persquare meter, act to a greater or lesser extent as a UV filter forincident light with respect to the fluorescent whitening agent presentin the base sheet.

In order to overcome these two problems, a number of solutions which aredescribed below have been proposed in the prior art and are used aloneor in combination.

A first solution, when a sheet of stationery is used, is to provide thefibrous base with a high fluorescence intensity which is then attenuatedto a greater or lesser extent depending on the composition and thicknessof the coat which is deposited on top. In practice, this pathway uses alarge quantity of fluorescent whitening agent and limits the coatdeposit to about 8 g/m² dry weight per face if a high fluorescenceintensity is to be retained.

Another solution is to introduce into the coat composition chemicalsubstances which act as a support for the fluorescent whitening agent,i.e. which act to fix the agent in a manner analogous to that ofcellulose. Support substances which may be cited include starch,polyvinyl alcohol (PVA), carboxymethylcellulose (CMC),polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), etc; they aremacromolecules which are rich in hydrophilic groups. In practice, thequantity of whitening agent is limited; with respect to the total dryweight of the pigments, it is usually 0.2% to less than 1% dry weight offluorescent whitening agent and 0.3% to 2% dry weight of supportsubstance. Indeed, if the quantity of fluorescent whitening agent is tobe increased, highly critical greening is observed beyond a value forΔa* of 1. The shade may be corrected by adding colorants, but thisreduces the luminosity, and thus the perception of whiteness. If thequantity of support substance is to be increased beyond 2% in order topush back this greening limit, another problem linked to the quality ofthe offset print (in particular with air-drying inks) is observed whichis due to a non-uniform ink-surface interaction, which produces anirregular, shadowed appearance in the print, termed “mottling”.

The aim of the present invention is to overcome the disadvantages of theprior art and the above alternatives to obtain coated sheets whichappear very white and which are offset-printable, in particular byair-drying offset inks without any substantial mottling defect.

The applicant remedies this by providing sheets of stationery and alsoplastic sheets, said sheets being coated and having a high fluorescenceintensity due to the incorporation of fluorescent whitening agents andsupport substances in relatively high quantities compared with thoseused in the prior art. The skilled person would not have been inclinedto use such quantities, for the reasons discussed above.

Thus, the invention provides a coated white sheet having a fluorescenceintensity of more than 55, measured in accordance with internationalstandard ISO 11475: 2004, by the difference between the value for theCIE whiteness under D65 illuminant and the value for this same CIEwhiteness after interposing a filter that eliminates wavelengths shorterthan 420 nm, and being offset-printable with no mottling, said coatedsheet comprising at least one base sheet and a printable white surfacecoat having a pigmented composition which comprises:

-   -   at least some white coating pigments and at least one coating        binder;    -   at least one fluorescent whitening agent in a total quantity of        1% dry weight or more with respect to the dry weight of said        pigments;    -   at least one support substance for said whitening agent in a        total quantity of dry weight of more than 2% with respect to the        dry weight of said pigments.

The fact that the coated sheet of the invention is “offset-printablewithout mottling” means that during offset-printing, in particular usingair-drying inks, it has no substantial surface mottling. This mottlingmay be evaluated, in particular, using the test described in theexamples below, which provides a mottling index.

Said coated sheet of the invention has a high fluorescence intensity andthus appears to be very white; further, its surface is offset-printable,in particular using air-drying inks, without substantial mottling.

More particularly, the Applicant has demonstrated that said surface coatmust have as regular a thickness as possible, even if it is highlyuneven below the surface of the base sheet and/or the sub-coats. Thisregularity of the coat deposit allows to obtain a homogeneous surfacecoat composition after drying and prevents mottling during printing. Infact, it has been shown that during drying of the coat that has beendeposited in an aqueous medium, the support substance, which is in facthydrosoluble, migrates in an irregular manner with the water as afunction of the irregularities of the coat deposit, these irregularitiesbeing notably linked to structural irregularities of the base onto whichit is applied and/or to some irregular penetration of the coat, whichthen during offset-printing (in particular with air-drying inks) causesa non-uniform ink-surface interaction which makes the print appearirregular, which may explain the mottling observed in the prior art whenthe support substances are used in a larger quantity.

In one particular case of the invention, said coated sheet may compriseat least one sub-coat comprising pigments and at least one binder,disposed below said surface coat.

More particularly, in accordance with the invention, the weight of saidsurface coat is at least 5 g/m² dry weight per face, preferably at least10 g/m², more particularly comprised between 15 and 30 g/m².

More particularly, in accordance with the invention, only this coatdeposited nearest the surface comprises a large quantity of fluorescentwhitening agent and support substance, the base sheet and/or thesub-coats possibly intrinsically having a low, or even zero fluorescenceintensity in order to reduce the cost of these fluorescent whiteningagents and support substances, which are expensive.

In fact, despite the low fluorescence intensity of the base and/or anypossible sub-coats, the Applicant has unexpectedly established that thesheets of the invention have a high fluorescence intensity, and thatthey do not exhibit substantial mottling on offset-printing.

Preferably, the total quantity of fluorescent whitening agent in saidsurface coat is more than 1% and less than or equal to 4% dry weightwith respect to the total dry weight of said coating pigments, inparticular comprised between 1.5% and 3%.

Preferably, the total quantity of support substance in said surface coatis 10% or less dry weight with respect to the total dry weight of saidcoating pigments, in particular comprised between 4% and 8%.

In accordance with a particular case of the invention, the totalquantity of fluorescent whitening agent in said surface coat is equal to1% dry weight with respect to the total dry weight of said coatingpigments, and the total quantity of support substance in said surfacecoat is more than 5% dry weight with respect to the total dry weight ofsaid coating pigments.

Preferably, said coated sheet of the invention has a fluorescenceintensity of more than 60, said intensity being measured usinginternational standard ISO 11475: 2004 as described above.

In accordance with a particular case of the invention, said base sheet,if appropriate coated with one or more sub-coats, has a fluorescenceintensity comprised between 0 and 20.

In accordance with a particular case of the invention, the coatingpigments of said coat are selected from calcium carbonates, kaolins,talcs, titanium dioxide and plastic pigments. As an example, the plasticpigments are hollow microspheres of a copolymer (styrene-acrylic) with amean size comprised between 1 μm and 0.1 μm.

More particularly, the surface coat comprises at least one coatingbinder selected from acrylic polymers, styrene-butadiene polymers andpossibly other monomers which are routinely used in coating, as well asother additives in common use such as shading colorants. The coatingbinders are used in the form of a stabilized aqueous dispersion (latex).

In accordance with the invention, the fluorescent whitening agent ismore particularly selected, alone or as a mixture, from variousderivatives of stilbene disulphonic acid with a total of 2, 4 or 6sulphonic groups, in particular those which are commercially available.They may possibly be commercial preparations already containing a verysmall quantity of a support substance.

In accordance with the invention, the support substance is selected,alone or as a mixture, from polyvinyl alcohol (PVA),carboxymethylcellulose (CMC), polyvinylpyrrolidone (PVP) and polymersbased on N-vinylformamide. The PVAs are preferably selected from thosewith a high degree of hydrolysis, in particular more than 98%.

The base sheet may be any fibrous sheet, in particular a sheet based oncellulose and/or synthetic fibres, such as a paper including board.

The base sheet may also be a sheet or a plastic film, for example apaper termed synthetic paper based on an extruded film of polyolefinfrom POLYART® or a sheet based on extruded polypropylene produced andsold by PRIPLAK®.

The base sheet may also be a complex of a fibrous sheet, in particular apaper and a film or plastic sheet or a paper coated with a coat ofextruded plastic.

The invention also concerns a process for producing said coated sheet.

The Applicant has also discovered that among the various means fordepositing a pigmented composition in an aqueous medium onto asubstrate, a contour coating process such as a curtain coating processcontributes to achieving the aims of the invention, in particular in thecase of a substrate with a surface which is not very regular. Indeed,this process allows to obtain a surface coat which has been depositedwith a very regular thickness thereby avoiding non-homogeneous migrationof the hydrosoluble support substance during drying, and avoidingmottling during offset-printing.

Thus, the invention also concerns a process for producing saidoffset-printable and white coated sheet with the characteristicsdescribed above in accordance with the invention, comprising a step forforming said surface coat by depositing the pigmented compositiondefining it, and carried out in an aqueous medium, by curtain coatingonto said base sheet, this latter being coated with sub-coat(s) ifappropriate.

More particularly, said coating process is applicable to a fibrous basesheet, in particular a paper. Curtain coating may be carried outon-machine during the fabrication of paper, or it may be carried outoff-machine.

In a particular case of the invention, the process is such that thepigmented composition of said surface coat is deposited simultaneouslywith that of a pigmented sub-coat described above using a multilayercurtain coating head.

However, in accordance with a particular case of the fabricationprocess, the said sub-coat or sub-coats may be applied using a sizingpress, for example on the paper machine on which the paper base isproduced or off-machine for a plastic based sheet. The sizing press maybe of the conventional type or it may be a modified model, for examplethe “Speedsizer” marketed by Voith. Other coating processes may be usedas long as they are appropriate for applying the desired weight ofcoats; several sub-coats may be applied using different processes. Afirst sub-coat may be coated using a sizing press and a second pigmentedsub-coat may be applied by blade coating, for example.

More particularly, the total dry weight of the set of coats of a sheetof the invention is more than 10 g/m² dry weight per face, preferably 15g/m², or even 30 g/m² per face or more.

The coated sheet of the invention may comprise said surface coat on eachof its faces.

The invention will be better understood with the aid of the followingnon-limiting or comparative examples and tests.

Tests

The fluorescence intensity is measured according to ISO 11475: 2004 bythe difference between the values of the CIE whiteness under D65illuminant and the value of this dimension measured after interposing afilter eliminating wavelengths below 420 nm.

By way of indication, we may mention measurements of the diffusereflectance factor in the blue R457 in accordance with standards ISO2469: 1994 and ISO 2470: 1999 under D65 illuminant as well as thewhitening intensity at 457 nm by difference with the value for thediffuse reflectance R457 after interposing a filter eliminatingwavelengths below 420 nm.

The greening Δa* is calculated by the difference between the value forthe trichromatic coordinate a* in the presence of a filter at 420 nm forthe coated sheet without fluorescent brightening agent and that underthe same measurement conditions for the coated sheet with a givenpercentage of fluorescent whitening agent.

The homogeneity of the print produced by offset-printing on a 4-colourROLAND 200 machine with air-drying inks NOVAFIT 918 SUPREME BIO fromFLINT GROUP GERMANY GmbH is evaluated by the mottling index, which isdetermined by image analysis using a KHEOPS machine sold by TECHPAP, theindex being on a scale of 1 (perfect) to 9 (very poor) with a score of 7or above being considered to be unacceptable for this use.

EXAMPLE 1

Example 1, along with Table 1, comprises comparative Examples 1a to 1cand Example 1d, in accordance with the invention.

A fibrous base (paper) which has previously been surfaced andpre-coated, the total coat weight being 30 g/m² dry weight per face,with pigmented compositions of calcium carbonate, starch binders andcopolymer binders (styrene-butadiene) and having a fluorescenceintensity of 39, is coated again in an amount of 17 g/m² dry weight perface with a pigmented surface composition produced in an aqueous mediumand essentially comprising:

-   -   a mixture of calcium carbonate (95 parts) and talc (5 parts)        pigments;    -   a copolymer binder (styrene-butadiene) in an amount of 7% dry        weight with respect to the dry pigment weight;    -   a fluorescent whitening agent A, which is a disulphone stilbene        derivative sold by 3Vsigma under the denomination OPTIBLANC NL,        in a quantity as a dry weight with respect to the total weight        of pigments as shown in Table 1;    -   a support substance for the whitening agent, which is a low        viscosity 4-98 type polyvinyl alcohol with a high degree of        hydrolysis in a quantity as a dry weight with respect to the        total weight of pigments as indicated in Table 1.

The surface compositions of comparative tests 1a to 1c are coated usingthe trailing blade (steel) process and the aqueous compositions areadjusted at identical dry matter contents (66%) and identicalviscosities.

The pigmented surface composition of test 1d of the invention isdeposited by curtain coating.

Thus, matt coated papers are obtained with a total coat weight of 47g/m² dry weight per face, as used for high quality offset sheet printing(sheet to sheet printing with air-drying inks) applications.

The coated papers of tests 1a to 1d are tested in offset-printing on the4 colour Roland machine and their mottling index is evaluated asdescribed above.

Comparative test 1a illustrates a standard coated paper foroffset-printing, both for its coat formulation and for its coatingprocess: it is established that the print quality is good, but thefluorescence intensity is insufficient.

Comparative tests 1b shows the negative effect of increasing thepercentage of fluorescent whitening agent which then results ingreening, which is the opposite effect to that desired.

Comparative test 1c shows that under the standard coating conditions itis not possible to increase the percentage of support substance withoutdegrading the printability (mottling) in an unacceptable manner.

Test 1d illustrates the invention; a coated sheet which appears verywhite is obtained because of the high fluorescence intensity and forwhich good offset print quality is observed with no notable mottling.

TABLE 1 Test 1a 1b 1c comparative comparative comparative 1d % support 11 6 6 substance % fluorescent 0.4 1.5 0.4 1.5 whitening agent A Coatingtype Blade Blade Blade Curtain Fluorescence 47 41 52 62 intensity ofcoated sheet Mottling 4 3.5 8 3

It should be noted that the coated paper 1d of the invention has a CIEwhiteness of 136, a diffuse reflectance factor in the blue R457 of 107%and a whitening intensity of 22%.

EXAMPLE 2

Example 2, along with Table 2, comprises comparative Examples 2a and 2band Examples 2c and 2d with surface compositions in accordance withthose defined for the invention.

Two aqueous pigmented compositions are prepared for matt coating, whichare constituted by the same mixture of calcium carbonate pigments and acopolymer binder (styrene butadiene) in an amount of 11% dry weight withrespect to the dry weight of the pigments.

One of the surface compositions, not in accordance with the invention,further comprises standard percentages according to the prior art (0.4%)of the support substance of Example 1 and a fluorescent whitening agentB which is a tetrasulphonated stilbene derivative sold by CLARIANT underthe denomination LEUCOPHOR LCPE.

The other composition comprises the same products but in quantitieswhich are in accordance with the invention.

These two compositions are each coated in an amount of 15 g/m² dryweight per face onto two fibrous bases (paper) of 120 g/m² with a low(16) or high (69) fluorescence intensity.

Table 2 shows that, in contrast to the surface compositions of theinvention (tests 2c and 2d), it is not possible with a standard coatingcomposition of the prior art (tests 2a and 2b) to reconcile a coatdeposit which is compatible with the printability requirement, i.e., acoat comprising a small quantity of support substance to avoid mottling,with a high fluorescence of the coated sheet (more than 55) even if apaper with a very high fluorescence intensity (test 2b) is used as thebase.

TABLE 2 Test 2a 2b 2c 2d Fluorescence intensity 16 69 16 69 of fibrousbase % support substance 0.4 6 % fluorescent 0.7 1.3 whitening agent BFluorescence intensity 45 51 59 63 of coated sheet

EXAMPLE 3

Example 3, along with Table 3, comprises comparative examples 3a and 3bwith surface compositions in accordance with those defined for theinvention.

These examples illustrate that even starting from a base with anear-zero fluorescence intensity, the pigmented compositions of theinvention allow to obtain coated sheets with a very high fluorescenceintensity.

Table 3 records the results obtained by coating a fibrous base with thesame pigmented composition as in Example 2, this time in the presence of6% (dry weight with respect to dry pigments) of the support substanceand 2.5% (dry weight with respect to dry pigments) of fluorescentwhitening agent B.

TABLE 3 Test 3a 3b Fluorescence intensity of 2 fibrous base Weight ofcoat (g/m² dry 15 30 weight per face) Fluorescence intensity of 62 69coated sheet

Coated papers 3a and 3b of the invention have respective whiteningintensity of 22% and 25%.

EXAMPLE 4

Example 4 comprises comparative examples 4a to 4d and Examples 4e to 4qwith surface compositions in accordance with those defined for theinvention.

These examples show that in contrast to the usual conditions, thesurface compositions in accordance with those defined in the inventiondo not have critical greening.

A fibrous base (paper) which has previously been surfaced andpre-coated, with a total coated weight of 15 g/m² dry weight per face,with pigmented compositions of calcium carbonate and a mixture of starchbinders and copolymer binders (styrene-butadiene), said pre-coated basehaving a fluorescence intensity of 43, is coated in an amount of 12 g/m²dry weight per face with a pigmented surface composition produced in anaqueous medium essentially comprising:

-   -   calcium carbonate pigments;    -   a copolymer binder (styrene-butadiene) in an amount of 8% dry        weight with respect to the dry pigment weight;    -   a fluorescent whitening agent in a quantity as a dry weight with        respect to the total weight of pigments as shown in Table 4;    -   a support substance in a dry weight quantity with respect to the        total weight of pigments as indicated in Table 4.

The support substance used is the one mentioned in the precedingexample; the fluorescent whitening agent can be agent A (disulphonatedstilbene derivative) or B (teirasulphonated stilbene derivative) alreadymentioned in the preceding examples, or another agent C which is ahexasulphonated stilbene derivative sold by ROBAMA under thedenomination RESISTOL SLK (see Table 4).

Demi-matt coated paper sheets are obtained.

Compared with comparative tests 4a to 4d, only tests 4e to 4q withsurface pigmented compositions in accordance with the invention haveboth a high fluorescence intensity and a greening Δa* of less than 1.

Depending on the mode of design of the coated sheet, one will select, onan industrial scale, the best quality/cost compromise which allows toachieve the desired fluorescence intensity with a greening Δa* of lessthan 1, preferably less than 0.6.

TABLE 4 Test 4a 4b 4c 4d 4e 4f 4g 4h 4i 4j 4k 4l 4m 4n 4o 4p 4q %support 0.8 3 6 9 substance % 0.4 1.5 2.5 4 1.5 1 1.5 2.5 4 1.5 2.5 4fluorescent whitening agent A % 1 1.5 2.5 4 fluorescent whitening agentB % 1.5 fluorescent whitening agent C Fluorescence 47 40 36 31 59 63 6463 59 58 63 66 68 65 66 67 63 intensity of coated sheet Δa* of test 0.240.70 1.21 1.72 0.59 0.38 0.52 0.64 0.94 0.26 0.34 0.39 0.47 0.24 0.420.57 0.79

EXAMPLE 5

This Example 5, along with Table 5, comprises Examples 5a to 5c whichillustrate the use of mixtures of support substances for the productionof pigmented compositions in accordance with those of the invention.

The coating conditions of Example 4 are repeated, with 2% (dry weightwith respect to dry weight of pigments) of fluorescent whitening agent A(disulphonated stilbene derivative), 0.8% (dry weight with respect todry weight of pigments) of PVA support substance as described in thepreceding examples and 5% (dry weight with respect to dry weight ofpigments) of one of the following support substances:

S1=CMC sold by HERCULES under the denomination BLANOSE 7L1C1;

S2=N-vinylformamide based polymer sold by BASF under the denominationLUPAMIN 4500;

S3=PVP sold by BASF under the denomination LUMITEN PPR 8450.

TABLE 5 Test 5a 5b 5c Support substance S1 S2 S3 Fluorescence intensity59 60 62 of coated sheet CIE whiteness of 141 142 144 coated sheet

It should be noted that the coated papers 5a, 5b and 5c of the inventionrespectively have a measure of reflectance factor R457 of 104% (for 5a),104.5% (for 5b) and 105% (for 5c), and a respective whitening intensityof 21% (for 5a), 21% (for 5b) and 22% (for 5c).

EXAMPLE 6

This Example 6, along with Table 6, illustrates the invention using aplastic sheet as the base sheet.

The coating composition of test 4o (9% of PVA support substance as a dryweight with respect to dry pigments and 1.5% dry weight with respect todry pigments of fluorescent whitening agent A (disulphonated stilbenederivative)) is applied at a coating weight of 23 g/m² dry weight perface deposited on a plastic sheet (based on extruded polyolefin filmcoated with a pigmented pre-coat) of 150 g/m² sold by ARJOBEX Ltd underthe trade name POLYART® with a fluorescence intensity of 8.

Table 6 shows that a sheet with a high fluorescence intensity isobtained.

TABLE 6 Test 6 Fluorescence intensity of coated sheet 68 Δa* for test0.6

The invention claimed is:
 1. A coated white sheet comprising at leastone base sheet and a printable and white surface coat, wherein the coathas a weight of from 10 to 30 g/m² dry weight per face, wherein the coathas a pigmented composition which comprises: white coating pigments andat least one coating binder; at least one fluorescence whitening agentin a total quantity of more than 1% dry weight with respect to the dryweight of said coating pigments; at least one support substance for saidwhitening agent in a total quantity of dry weight of more than 2% withrespect to the dry weight of said coating pigments, and wherein the coathas a regular thickness, so that the coated white sheet has afluorescence intensity of more than 55, measured in accordance withinternational standard ISO 11475: 2004, by the difference between thevalue for the CIE whiteness under D65 illuminant and the value for thissame CIE whiteness after interposing a filter that eliminateswavelengths shorter than 420 nm, and the coated white sheet isoffset-printable with no mottling.
 2. A coated sheet according to claim1, which comprises at least one sub-coat comprising pigments and atleast one binder, disposed below said surface coat.
 3. A coated sheetaccording to claim 1, wherein the total quantity of dry weight of saidfluorescent whitening agent in said surface coat is greater than 1% andlower than 4% dry weight with respect to the dry weight of said coatingpigments.
 4. A coated sheet according to claim 1, wherein the totalquantity of dry weight of said support substance in said surface coat isgreater than 2% and lower than 10% dry weight with respect to the dryweight of said coating pigments.
 5. A coated sheet according to claim 1,wherein the coating pigments of said surface coat are selected from thegroup consisting of calcium carbonates, kaolins, talcs, and titaniumdioxide.
 6. A coated sheet according to claim 1, wherein saidfluorescent whitening agent of said surface coat is selected from thegroup consisting of stilbene derivatives containing a total of 2, 4 or 6sulphonic groups, and mixtures thereof.
 7. A coated sheet according toclaim 1, wherein said support substance is selected from the groupconsisting of polyvinyl alcohols, carboxymethylcellulose,polyvinylpyrrolidone, polymers based on N-vinylformamide, and mixturesthereof.
 8. A coated sheet according to claim 7, wherein the polyvinylalcohols have a degree of hydrolysis higher than 98%.
 9. A coated sheetaccording to claim 1, wherein said surface coat comprises a coatingbinder selected from the group consisting of acrylic polymers,styrene-butadiene polymers, and mixtures thereof.
 10. A coated sheetaccording to claim 1, wherein said fluorescence intensity of said coatedsheet is more than
 60. 11. A coated sheet according to claim 1, whereinsaid base sheet has a fluorescence intensity comprised between 0 and 20.12. A coated sheet according to claim 1, wherein said base sheet is aone of a fibrous sheet, a film, a plastic sheet, a composite of a filmand a fibrous sheet, or a composite of a plastic sheet and a fibroussheet.
 13. The coated white sheet according to claim 1, wherein thetotal quantity of dry weight of the support substance is at least 6%with respect to the dry weight of said coating pigments.
 14. The coatedwhite sheet according to claim 1, wherein the surface coat has a weightof from 12 to 30 g/m² dry weight per face and the total quantity of dryweight of the support substance is at least 3% with respect to the dryweight of said coating pigments.
 15. The coated white sheet according toclaim 1, wherein the surface coat has a weight of from 12 to 30 g/m² dryweight per face and the total quantity of dry weight of the supportsubstance is at least 6% with respect to the dry weight of said coatingpigments.
 16. A process so as to obtain a coated sheet according toclaim 1, wherein said surface coat is formed by depositing its saidpigmented composition in an aqueous medium by curtain coating onto saidbase sheet, if appropriate coated with sub-coats.
 17. A processaccording to claim 16, wherein the pigmented composition of said surfacecoat is deposited by curtain coating simultaneously with a pigmentedsub-coat.
 18. A process according to claim 16, wherein the base sheet isa paper.
 19. A process according to claim 17, wherein the base sheet isa paper.